1. Field of the Invention
The present invention is related to a high-temperature-resistant metal-packaged fiber Bragg grating sensor and manufacturing method thereof.
2. Description of Related Art
Optical fiber sensors are used for detecting changes in light transmission characteristics when light propagates through optical fibers being modulated by the environmental effects (physical parameters, chemical parameters, biological parameters, etc.) where all or parts of the optical fibers are located. Optical fiber sensors are widely used in various industries due to their many advantages, such as small size, light weight, high accuracy and sensitivity, immunity to electromagnetic interference and radiation, corrosion resistance, fire prevention, explosion proof, and long service life.
Fiber grating sensor based on wavelength modulation is a type of optical fiber sensors. A fiber grating sensor is to monitor a shift in wavelength which is sensitive to changes in the measurand (physical parameters, chemical parameters, biological parameters, etc). In addition to all the advantages attributed to optical fiber sensors, wavelength encoded fiber grating sensors have an inherent self-referencing capability and a wavelength multiplexing capacity. Accordingly, they can be easily multiplexed in a single fiber spliced to telecommunication fibers for distributed sensing, remote sensing and multi-parameter sensing. Also, the fiber grating sensors can be distributed embedded into materials to create “smart materials”. At present, the fiber grating sensors have a wide range of applications in civil engineering, wind power, composite materials, etc.
The most common fiber Bragg grating (FBG) is the most widely used fiber grating, which consists of a periodic modulation of the refractive index in the core of a short segment of a single-mode optical fiber. These types of uniform fiber gratings, where the phase fronts are perpendicular to the fiber longitudinal axis, have constant grating pitch and index modulation amplitude. Light guided along the core of an optical fiber will be scattered by each grating plane. If the Bragg condition is not satisfied, the reflected light from each of the subsequent planes becomes progressively out of phase and will eventually cancel out. Where the Bragg condition is satisfied, the contributions of reflected light from each grating plane add constructively in the backward direction to form a back-reflected peak with a center wavelength namely Bragg wavelength. According to Maxwell's equations and the coupled-mode theory, the Bragg wavelength λB can be expressed asλB=2neffΛ  (1)where neff is the effective refractive index of fiber core, and Λ is the grating pitch.
According to the equation (1), the Bragg wavelength λB depends on the effective refractive index neff of the fiber core and the periodicity of the grating A. The effective refractive index, as well as the periodic pitch between the Bragg grating planes, will be affected by changes in temperature and strain. The change in the effective refractive index neff is related to the thermo-optic effect and stain-optic effect induced by changes in temperature and strain, as well as the change in grating pitch Λ is related to thermal expansion and mechanical deformation induced by changes in temperature and strain. Accordingly, the measurement of temperature, stress/strain can be achieved by monitoring the changes in the Bragg wavelength λB for structural integrity monitoring.
Up to now, many research projects and applications of fiber Bragg grating sensors have been undertaken in the fields of civil engineering, wind power, composite materials, etc., where fiber Bragg grating sensors is relatively easy to be attached onto surfaces of components or embed into them due to fabrication and operation of the components at relative low temperature. However, in fields of nuclear power, thermal power, petrochemical industry, aerospace industry and so on, where measured metallic components are mostly operated at high temperature, common fiber Bragg grating sensors have obvious disadvantages as follows:
1) Reflectivity of conventional fiber Bragg gratings start to decay when the temperature is above 200° C. and the gratings are “erased” completely when up to 680° C. So conventional fiber Bragg grating sensors can only be used below 200° C.
2) During fabrication of gratings, polymer coatings on surfaces of the optical fibers are always removed so that the optical fibers are exposed to moisture and easily to be mechanically damaged resulting in crack formation on the surfaces, which causes the strength of optical fiber decreases. So the optical fibers must be recoated and packaged after inscribing gratings. Normally, it is mainly organic polymer materials which are used to recoat and packaged fiber Bragg gratings, for example, Chinese patent application No. 201110135194.2 discloses a metal-packaged fiber grating sensor and manufacturing method thereof. The disadvantages of polymer materials, such as ageing and creep, restrict the performance of sensors thus hard to survive in the damp and hot environment. When it is above their normal work temperature, the polymer materials will soften and even decompose to generate hydrogen gas which has a stress corrosion effect on silica optical fibers thereby accelerate fatigue of the silica optical fibers. When it is above 400° C., the polymer materials decompose completely so that optical fibers lose their protection in harsh environment and easily form cracks on their surfaces under thermal-mechanical loading. As time goes by, the cracks grow slowly which results in a decrease in the strength and eventually causes fracture of the optical fiber. Apparently, it is hard to monitor high-temperature components for a long time based on conventional fiber Bragg grating sensors.
3) Conventional fiber Bragg grating sensors and components to be monitored are usually connected by organic adhesive. While, the organic adhesive will produce measurement redundancy, lower strain transfer efficiency, poor linearity, poor repeatability and poor long-term reliability. With increasing in temperature, the organic adhesive accelerates its ageing and further starts to soften and even decompose when up to more than 250° C. Thus the connection between the sensors and the high-temperature components could hardly be achieved.
4) Low thermo-optic coefficient and thermal expansion coefficient of silica (SiO2) optical fiber result in the low temperature sensitivity of bare fiber Bragg grating sensors.
Accordingly, for effectively monitoring metallic components at high temperature for a long time, not only fiber Bragg gratings itself but also metallic package should survive at high temperatures to achieve. So that it can be embedded into metallic components to be monitored or attached on surfaces of them by welding.
To meet the requirements of application in high temperature, many techniques have been proposed to increase the thermal stability of the gratings, including inscribing gratings in specially doped fibers, etching the grating into the flat surface of a D-shaped optical fiber, inscribing gratings with femtosecond laser, etc. However, the specially doped optical fibers are expensive, etching gratings on surfaces requires high accuracy and the corresponding packaging is very difficult since the grating is etched on the cladding and the femtosecond laser costs too much.
Currently, it is commonly used to achieve the metallization of optical fibers including casting, laser cladding, electroless plating, combination of electroless plating and electroplating, vacuum evaporation and so on. Casting and laser cladding have a lot of restriction on coated metals, wherein metals with low melting points cannot meet the requirement of high temperature application; and metals with high melting points have too high melting temperature under which optical fiber gratings are easily damaged and would cause high thermal stress leading to fracture of optical fibers. Furthermore, both of the two methods cannot ensure a metallic coating uniformly deposited on the surface of a grating along an axial direction thereof so that polarization phenomenon occurs and influences spectral shape. The electroless plating methods are provided by, for example, CN200410061378.9 named by “a metalized packaging structure of an optical fiber sensitive element and method thereof”, CN200510020086.5 named by “a wet-chemical metallization process of a surface of a silica optical fiber”, and CN201010504623.4 named by “a method of electroless plating on a surface of a silica optical fiber”. The combination methods of electroless plating and electroplating are provided by, for example, CN02816378.8 named by “a metal-plated optical fiber”, and CN03804115.4 named by “a metal-coated optical fiber”. The bonding between the coatings obtained from electroless plating and the optical fibers is poor and the coatings have poor uniformity, which cannot meet the requirements of high sensitivity sensors. Moreover, the coarsening and sensitizing process during pretreatment of the electroless plating will damage surfaces of the optical fibers and reduce strength of the optical fibers. Further, the strength of the optical fibers decreases significantly since the optical fibers are directly exposed to corrosive plating solutions containing water, acid and alkali during electroless plating. A bonding force between coating and a substrate surface is weak during vacuum evaporation, and it is difficult to deposit a coating with a high melting point under low vapor pressure, and crucible materials for evaporating substance will evaporate and become impurities mixed into the coating. Also, the coating obtained from vacuum evaporation is too thin to sufficiently protect the optical fibers and transfer strain from the measured components to optical fibers.